The present invention relates to a device for measuring forces and pressures which lead to the deformation of an oscillating piezoelectric membrane, whereby the change in the natural frequency of this membrane constitutes a measure of the size of the deformation and, consequently, a measure of the size of the force causing the deformation.
There are many applications for force transducers in science and technology. On the one hand, these transducers are used for direct force measurement in scales and accelerometers while, on the other hand, they are also widely applied for measuring the pressure of fluids and gases.
It is known that certain natural or artificial crystals can be used as force transducers based on piezoelectric excitation. An exceptionally high elasticity accompanied by practically negligible hysteresis, as well as the chemical stability of the single-crystal structure, make quartz a suitable basic material for such transducers with long-term stability.
As a function of its cut and geometric shape, quartz can be excited piezoelectrically and produce natural oscillations which, in the case of resonance, possess a high Q value (in an oscillating system this is the ratio of energy stored to energy lost during one oscillation). This permits quartz to operate in an oscillator circuit with a weak or low-power, external energy source. If the internal energy of the quartz changes, for example, due to a change in temperature or due to mechanical deformation, with no actual change in the Q value of the system, that is, there is no sizable attenuation of the system, the result will be a change in the natural frequency of resonance of the quartz crystal. Consequently, the frequency will be a direct measure of, for example, the deformation of the quartz, and the signal can be further processed as a digital signal without additional conversion.
Therefore, the design of a quartz resonating force and pressure transducer must be such that the externally operating force does not actually change the Q value of the system. For that reason, the mounting of the quartz resonating force and pressure transducer is very important. Moreover, one has to make sure that it does not affect the oscillation ratio of the quartz.
These necessary conditions for a quartz resonating force and pressure transducer (QRFPT) with resonance oscillations could, until now, only be met by the external design of the quartz and the mounting system.
Some examples of quartz resonating force and pressure transducers can be found in the following U.S. Pat. Nos.: 3,399,572; 3,470,400; 3,479,536; 3,505,866; 4,020,448; 4,067,241; 4,091,679; and 4,104,920. The quartz resonating force and pressure transducers according to U.S. Pat. Nos. 4,215,279; and 4,372,173 are somewhat easier to operate.
Nevertheless, these transducers also present mounting problems, in particular with regard to the occurrence of undesired oscillations.